The Marine Life Information Network

Global warming (extreme)

Extreme emission scenario (by the end of this century 2081-2100) benchmark of:

• A 5°C rise in SST and NBT (coastal to the shelf seas),

• A 6°C rise in surface air temperature (in eulittoral and supralittoral habitats).

• A 1°C rise in Deep-sea habitats (>200 m) off the continental shelf, and

• A 5°C rise in surface air temperature in intertidal habitats exclusive to Scotland. Further detail.

Evidence

Sea surface temperatures around the UK currently fall between 6 to 19°C (Huthnance, 2010). Under the middle emission, high emission and extreme scenarios, sea surface temperatures are expected to increase by 3, 4 and 5°C respectively, leading to temperatures increasing to between 22 to 24°C by the end of this century, although northern UK temperatures will be up to 5°C lower. 

Gili & Hughes (1995) reported that temperature is a critical factor stimulating or preventing reproduction in hydroids and that most species have an optimal temperature for reproduction. However, limited evidence on the thermal thresholds and thermal ranges were available for the characterizing species recorded in this biotope and there was no direct evidence of the impact of ocean warming.

In the Mediterranean Sea, increasing sea temperatures are leading to changes in hydroid community composition, with shallow water species increasing their depth range and warm water species moving northwards (Puce et al., 2009). Non-native hydroids have spread (Bethencourt et al., 2013) in the Mediterranean, whereas previously present native species have disappeared (Puce et al., 2009, Gravili et al., 2015).

Sertularia cupressina is found across the north-west European shelf and on the west coast of the North America coastline from Canada to as far south as Virginia, where sea surface temperatures average 26°C in summer months, reaching temperatures of up to 30°C (www.seatemperature.org). In the Bay of Fundy, the reproductive period was reported to occur between in the warm water months of May and October (Henry & Kenchington, 2004), where sea temperature ranges between 2 and 14°C, whereas in Langstone Harbour, UK it occurs in winter between November to January (Schmidt & Warner, 1991) where temperatures range from 14°C to 7°C.  This observation suggests that reproduction is triggered by temperature, rather than season. Hydrallmania falcata shows a similar distribution and reproductive pattern; and is found across the north-west European shelf and along the west coast of North America coastline (www.obis.org). In the UK it is thought to reproduce during the coldest months, between December to April (MBA, 1957).

Flustra foliacea is a perennial cold temperate species (Stebbing, 1971a), which is widespread throughout the British Isles and North Atlantic (NBN, 2015). Stebbing (1971a) noted that Flustra foliacea on the Gower peninsular, South Wales had an annual growth season between March and November. It is also distributed across north-west Europe (Fish & Fish, 1996), occurring in the Kara Sea, White Sea and Barents Sea in the Arctic circle, the North Sea, on the east coast of Greenland and extends south as far as Bay of Biscay. OBIS (2024) lists records of Flustra foliacea from sea surface temperatures of 0 to 15°C. The predominantly northern distribution of Flustra foliacea could suggest it is likely to move further northward as the climate changes and temperature shifts out of its preferred range, particularly as it is a cold temperate species. Elevated seawater temperatures generally increase the metabolic rate of bryozoans such as Flustra foliacea and is expected to affect calcification (Smith, 2014; Moreno, 2020). There is no further information available on the temperature tolerance of Flustra foliacea.

Sensitivity assessment. Both Hydrallmania falcata and Sertularia cupressina are known to live along the west coast of North America, at least as far south as Virginia and possibly extending into the states of Carolina (www.obis.org), and are, therefore, able to withstand high summer temperatures of up to 30°C.  However,  reproduction occurs during the winter months in southern populations, suggesting that increasingly warm winter temperatures may be the limiting factor for reproduction. On the other hand, Flustra foliacea is a cold temperate species recorded at temperatures up to 15°C (OBIS, 2024). It is possible that higher temperatures could exceed the species thermal limit. In the south of the UK, minimum winter sea surface temperature is 9°C, which may increase to between 12 to 14°C by the end of this century, dependent on emissions scenario, with temperatures potentially exceeding a mean monthly temperature of 14°C between January to March under both the high emission and the extreme scenario, which may suppress reproduction in both species. Therefore, it is likely that reproductive output may decline in southern populations under the middle and high emission scenarios, due to increasingly warm winter temperatures, leading to a population decline. Hence, resistance is assessed as ‘Medium’. Due to the long-term nature of ocean warming, recovery has been assessed as ‘Very low’, leading to an overall sensitivity assessment of ‘Medium’. Under the extreme scenario, there is potential that an increase in winter sea temperatures of 14°C, which appears to be the threshold temperature for reproduction in both species, could lead to further reductions in recruitment, negatively impacting the persistence of these species. Therefore, resistance has been assessed as ‘Low’, resilience as ‘Very low’ and sensitivity assessed as ‘High’, but with ‘Low’ confidence.

Global warming (high)

High emission scenario (by the end of this century 2081-2100) benchmark of:

• A 4°C rise in SST, NBT (coastal to the shelf seas) and surface air temperature (in eulittoral and supralittoral habitats).

• A 1°C rise in Deep-sea habitats (>200 m) off the continental shelf, and

• A 3°C rise in surface air temperature in intertidal habitats exclusive to Scotland. Further detail.

Evidence

Sea surface temperatures around the UK currently fall between 6 to 19°C (Huthnance, 2010). Under the middle emission, high emission and extreme scenarios, sea surface temperatures are expected to increase by 3, 4 and 5°C respectively, leading to temperatures increasing to between 22 to 24°C by the end of this century, although northern UK temperatures will be up to 5°C lower. 

Gili & Hughes (1995) reported that temperature is a critical factor stimulating or preventing reproduction in hydroids and that most species have an optimal temperature for reproduction. However, limited evidence on the thermal thresholds and thermal ranges were available for the characterizing species recorded in this biotope and there was no direct evidence of the impact of ocean warming.

In the Mediterranean Sea, increasing sea temperatures are leading to changes in hydroid community composition, with shallow water species increasing their depth range and warm water species moving northwards (Puce et al., 2009). Non-native hydroids have spread (Bethencourt et al., 2013) in the Mediterranean, whereas previously present native species have disappeared (Puce et al., 2009, Gravili et al., 2015).

Sertularia cupressina is found across the north-west European shelf and on the west coast of the North America coastline from Canada to as far south as Virginia, where sea surface temperatures average 26°C in summer months, reaching temperatures of up to 30°C (www.seatemperature.org). In the Bay of Fundy, the reproductive period was reported to occur between in the warm water months of May and October (Henry & Kenchington, 2004), where sea temperature ranges between 2 and 14°C, whereas in Langstone Harbour, UK it occurs in winter between November to January (Schmidt & Warner, 1991) where temperatures range from 14°C to 7°C.  This observation suggests that reproduction is triggered by temperature, rather than season. Hydrallmania falcata shows a similar distribution and reproductive pattern; and is found across the north-west European shelf and along the west coast of North America coastline (www.obis.org). In the UK it is thought to reproduce during the coldest months, between December to April (MBA, 1957).

Flustra foliacea is a perennial cold temperate species (Stebbing, 1971a), which is widespread throughout the British Isles and North Atlantic (NBN, 2015). Stebbing (1971a) noted that Flustra foliacea on the Gower peninsular, South Wales had an annual growth season between March and November. It is also distributed across north-west Europe (Fish & Fish, 1996), occurring in the Kara Sea, White Sea and Barents Sea in the Arctic circle, the North Sea, on the east coast of Greenland and extends south as far as Bay of Biscay. OBIS (2024) lists records of Flustra foliacea from sea surface temperatures of 0 to 15°C. The predominantly northern distribution of Flustra foliacea could suggest it is likely to move further northward as the climate changes and temperature shifts out of its preferred range, particularly as it is a cold temperate species. Elevated seawater temperatures generally increase the metabolic rate of bryozoans such as Flustra foliacea and is expected to affect calcification (Smith, 2014; Moreno, 2020). There is no further information available on the temperature tolerance of Flustra foliacea.

Sensitivity assessment. Both Hydrallmania falcata and Sertularia cupressina are known to live along the west coast of North America, at least as far south as Virginia and possibly extending into the states of Carolina (www.obis.org), and are, therefore, able to withstand high summer temperatures of up to 30°C.  However,  reproduction occurs during the winter months in southern populations, suggesting that increasingly warm winter temperatures may be the limiting factor for reproduction. On the other hand, Flustra foliacea is a cold temperate species recorded at temperatures up to 15°C (OBIS, 2024). It is possible that higher temperatures could exceed the species thermal limit. In the south of the UK, minimum winter sea surface temperature is 9°C, which may increase to between 12 to 14°C by the end of this century, dependent on emissions scenario, with temperatures potentially exceeding a mean monthly temperature of 14°C between January to March under both the high emission and the extreme scenario, which may suppress reproduction in both species. Therefore, it is likely that reproductive output may decline in southern populations under the middle and high emission scenarios, due to increasingly warm winter temperatures, leading to a population decline. Hence, resistance is assessed as ‘Medium’. Due to the long-term nature of ocean warming, recovery has been assessed as ‘Very low’, leading to an overall sensitivity assessment of ‘Medium’. Under the extreme scenario, there is potential that an increase in winter sea temperatures of 14°C, which appears to be the threshold temperature for reproduction in both species, could lead to further reductions in recruitment, negatively impacting the persistence of these species. Therefore, resistance has been assessed as ‘Low’, resilience as ‘Very low’ and sensitivity assessed as ‘High’, but with ‘Low’ confidence.

Global warming (middle)

Middle emission scenario (by the end of this century 2081-2100) benchmark of:

• A 3°C rise in SST, NBT (coastal to the shelf seas) and surface air temperature (in eulittoral and supralittoral habitats).

• A 1°C rise in Deep-sea habitats (>200 m) off the continental shelf.

• A 2°C rise in surface air temperature in intertidal habitats exclusive to Scotland. Further detail.

Evidence

Sea surface temperatures around the UK currently fall between 6 to 19°C (Huthnance, 2010). Under the middle emission, high emission and extreme scenarios, sea surface temperatures are expected to increase by 3, 4 and 5°C respectively, leading to temperatures increasing to between 22 to 24°C by the end of this century, although northern UK temperatures will be up to 5°C lower. 

Gili & Hughes (1995) reported that temperature is a critical factor stimulating or preventing reproduction in hydroids and that most species have an optimal temperature for reproduction. However, limited evidence on the thermal thresholds and thermal ranges were available for the characterizing species recorded in this biotope and there was no direct evidence of the impact of ocean warming.

In the Mediterranean Sea, increasing sea temperatures are leading to changes in hydroid community composition, with shallow water species increasing their depth range and warm water species moving northwards (Puce et al., 2009). Non-native hydroids have spread (Bethencourt et al., 2013) in the Mediterranean, whereas previously present native species have disappeared (Puce et al., 2009, Gravili et al., 2015).

Sertularia cupressina is found across the north-west European shelf and on the west coast of the North America coastline from Canada to as far south as Virginia, where sea surface temperatures average 26°C in summer months, reaching temperatures of up to 30°C (www.seatemperature.org). In the Bay of Fundy, the reproductive period was reported to occur between in the warm water months of May and October (Henry & Kenchington, 2004), where sea temperature ranges between 2 and 14°C, whereas in Langstone Harbour, UK it occurs in winter between November to January (Schmidt & Warner, 1991) where temperatures range from 14°C to 7°C.  This observation suggests that reproduction is triggered by temperature, rather than season. Hydrallmania falcata shows a similar distribution and reproductive pattern; and is found across the north-west European shelf and along the west coast of North America coastline (www.obis.org). In the UK it is thought to reproduce during the coldest months, between December to April (MBA, 1957).

Flustra foliacea is a perennial cold temperate species (Stebbing, 1971a), which is widespread throughout the British Isles and North Atlantic (NBN, 2015). Stebbing (1971a) noted that Flustra foliacea on the Gower peninsular, South Wales had an annual growth season between March and November. It is also distributed across north-west Europe (Fish & Fish, 1996), occurring in the Kara Sea, White Sea and Barents Sea in the Arctic circle, the North Sea, on the east coast of Greenland and extends south as far as Bay of Biscay. OBIS (2024) lists records of Flustra foliacea from sea surface temperatures of 0 to 15°C. The predominantly northern distribution of Flustra foliacea could suggest it is likely to move further northward as the climate changes and temperature shifts out of its preferred range, particularly as it is a cold temperate species. Elevated seawater temperatures generally increase the metabolic rate of bryozoans such as Flustra foliacea and is expected to affect calcification (Smith, 2014; Moreno, 2020). There is no further information available on the temperature tolerance of Flustra foliacea.

Sensitivity assessment. Both Hydrallmania falcata and Sertularia cupressina are known to live along the west coast of North America, at least as far south as Virginia and possibly extending into the states of Carolina (www.obis.org), and are, therefore, able to withstand high summer temperatures of up to 30°C.  However,  reproduction occurs during the winter months in southern populations, suggesting that increasingly warm winter temperatures may be the limiting factor for reproduction. On the other hand, Flustra foliacea is a cold temperate species recorded at temperatures up to 15°C (OBIS, 2024). It is possible that higher temperatures could exceed the species thermal limit. In the south of the UK, minimum winter sea surface temperature is 9°C, which may increase to between 12 to 14°C by the end of this century, dependent on emissions scenario, with temperatures potentially exceeding a mean monthly temperature of 14°C between January to March under both the high emission and the extreme scenario, which may suppress reproduction in both species. Therefore, it is likely that reproductive output may decline in southern populations under the middle and high emission scenarios, due to increasingly warm winter temperatures, leading to a population decline. Hence, resistance is assessed as ‘Medium’. Due to the long-term nature of ocean warming, recovery has been assessed as ‘Very low’, leading to an overall sensitivity assessment of ‘Medium’. Under the extreme scenario, there is potential that an increase in winter sea temperatures of 14°C, which appears to be the threshold temperature for reproduction in both species, could lead to further reductions in recruitment, negatively impacting the persistence of these species. Therefore, resistance has been assessed as ‘Low’, resilience as ‘Very low’ and sensitivity assessed as ‘High’, but with ‘Low’ confidence.

Marine heatwaves (high)

High emission scenario benchmark: A marine heatwave occurring every two years, with a mean duration of 120 days, and a maximum intensity of 3.5°C. Further detail.

Evidence

Marine heatwaves due to increased air-sea heat flux are predicted to occur more frequently, last for longer and at increased intensity by the end of this century under both middle and high emission scenarios (Frölicher et al., 2018). Climate change will not only shift mean sea surface temperatures but will also increase the intensity of extreme events, exerting additional stress on ecosystems.

Both Hydrallmania falcata and Sertularia cupressina occur along the west coast of North America from Canada to as far south as Virginia, where sea surface temperatures average 26°C in summer months, reaching temperatures of up to 30°C (www.seatemperature.org), suggesting they may be able to withstand high summer temperatures. Reproduction in both species is thought to be limited to cooler temperatures, with both species reproducing during winter months at their southern geographical distributions (MBA, 1957, Schmidt & Warner, 1991). Reproduction in these species may already be suppressed by rising sea surface temperatures (see global warming above), and whilst a degree of tolerance to summer heatwaves may be possible, winter heatwaves may lead to suppression of reproduction in both species.

Flustra foliacea is a perennial cold temperate species (Stebbing, 1971a), which is widespread throughout the British Isles and North Atlantic (NBN, 2015). Stebbing (1971a) noted that Flustra foliacea on the Gower peninsular, South Wales had an annual growth season between March and November. It is also distributed across north-west Europe (Fish & Fish, 1996), occurring in the Kara Sea, White Sea and Barents Sea in the Arctic circle, the North Sea, on the east coast of Greenland and extends south as far as Bay of Biscay. OBIS (2024) lists records of Flustra foliacea from sea surface temperatures of 0 to 15°C. The predominantly northern distribution of Flustra foliacea could suggest it is likely to move further northward as the climate changes and temperature shifts out of its preferred range, particularly as it is a cold temperate species. Elevated seawater temperatures generally increase the metabolic rate of bryozoans such as Flustra foliacea and is expected to affect calcification (Smith, 2014; Moreno, 2020). There is no further information available on the upper thermal limits of Flustra foliacea.

Sensitivity assessment. Under the middle emission scenario, if heatwaves occurred at a frequency of every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to sea temperatures reaching up to 24°C in southern England in summer months. Under the high emission scenario, if heatwaves occur at a frequency of every two years by the end of this century, with a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C. Both Hydrallmania falcata and Sertularia cupressina will likely be able to withstand heatwaves of these intensities and durations, as they can withstand temperatures of up to 30°C at their southern geographical limit. If these heatwaves occurred in the winter months, they could lead to winter temperatures rising to ≥15°C, which could potentially lead to suppression of sexual reproduction, although an episodic reduction in sexual reproduction is not likely to lead to long-term negative impacts. Flustra foliacea is a cold temperate species recorded at temperatures up to 15°C (OBIS, 2024), it is possible that higher temperatures could exceed the species thermal limit. This suggests that Flustra foliacea may not be tolerant to heatwaves. Therefore, under both the middle and high emission scenario resistance is assessed as ‘Medium’. Hydroids and bryozoans are capable of rapid colonization and growth and are likely to recover under the middle scenario with three years between heatwaves so resilience is assessed as ‘High', and sensitivity as ‘Low’. However, if heatwaves occurred every two years (the high emission scenario) Flustra may not have time to recolonize affected areas before the next heatwave, leading to population decline, especially in southern waters, so resilience is assessed as ‘Very low’ and sensitivity as ‘Medium’. Note, confidence in the assessments are ‘Low’ due to the lack of direct evidence.

Marine heatwaves (middle)

Middle emission scenario benchmark:  A marine heatwave occurring every three years, with a mean duration of 80 days, with a maximum intensity of 2°C. Further detail.

Evidence

Marine heatwaves due to increased air-sea heat flux are predicted to occur more frequently, last for longer and at increased intensity by the end of this century under both middle and high emission scenarios (Frölicher et al., 2018). Climate change will not only shift mean sea surface temperatures but will also increase the intensity of extreme events, exerting additional stress on ecosystems.

Both Hydrallmania falcata and Sertularia cupressina occur along the west coast of North America from Canada to as far south as Virginia, where sea surface temperatures average 26°C in summer months, reaching temperatures of up to 30°C (www.seatemperature.org), suggesting they may be able to withstand high summer temperatures. Reproduction in both species is thought to be limited to cooler temperatures, with both species reproducing during winter months at their southern geographical distributions (MBA, 1957, Schmidt & Warner, 1991). Reproduction in these species may already be suppressed by rising sea surface temperatures (see global warming above), and whilst a degree of tolerance to summer heatwaves may be possible, winter heatwaves may lead to suppression of reproduction in both species.

Flustra foliacea is a perennial cold temperate species (Stebbing, 1971a), which is widespread throughout the British Isles and North Atlantic (NBN, 2015). Stebbing (1971a) noted that Flustra foliacea on the Gower peninsular, South Wales had an annual growth season between March and November. It is also distributed across north-west Europe (Fish & Fish, 1996), occurring in the Kara Sea, White Sea and Barents Sea in the Arctic circle, the North Sea, on the east coast of Greenland and extends south as far as Bay of Biscay. OBIS (2024) lists records of Flustra foliacea from sea surface temperatures of 0 to 15°C. The predominantly northern distribution of Flustra foliacea could suggest it is likely to move further northward as the climate changes and temperature shifts out of its preferred range, particularly as it is a cold temperate species. Elevated seawater temperatures generally increase the metabolic rate of bryozoans such as Flustra foliacea and is expected to affect calcification (Smith, 2014; Moreno, 2020). There is no further information available on the upper thermal limits of Flustra foliacea.

Sensitivity assessment. Under the middle emission scenario, if heatwaves occurred at a frequency of every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to sea temperatures reaching up to 24°C in southern England in summer months. Under the high emission scenario, if heatwaves occur at a frequency of every two years by the end of this century, with a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C. Both Hydrallmania falcata and Sertularia cupressina will likely be able to withstand heatwaves of these intensities and durations, as they can withstand temperatures of up to 30°C at their southern geographical limit. If these heatwaves occurred in the winter months, they could lead to winter temperatures rising to ≥15°C, which could potentially lead to suppression of sexual reproduction, although an episodic reduction in sexual reproduction is not likely to lead to long-term negative impacts. Flustra foliacea is a cold temperate species recorded at temperatures up to 15°C (OBIS, 2024), it is possible that higher temperatures could exceed the species thermal limit. This suggests that Flustra foliacea may not be tolerant to heatwaves. Therefore, under both the middle and high emission scenario resistance is assessed as ‘Medium’. Hydroids and bryozoans are capable of rapid colonization and growth and are likely to recover under the middle scenario with three years between heatwaves so resilience is assessed as ‘High', and sensitivity as ‘Low’. However, if heatwaves occurred every two years (the high emission scenario) Flustra may not have time to recolonize affected areas before the next heatwave, leading to population decline, especially in southern waters, so resilience is assessed as ‘Very low’ and sensitivity as ‘Medium’. Note, confidence in the assessments are ‘Low’ due to the lack of direct evidence.

Ocean acidification (high)

High emission scenario benchmark: a further decrease in pH of 0.35 (annual mean) and corresponding 120% increase in H+ ions , seasonal aragonite saturation of 20% of UK coastal waters and North Sea bottom waters, and the aragonite saturation horizon in the NE Atlantic, off the continental shelf, occurring at a depth of 400 m by the end of this century 2081-2100. Further detail 

Evidence

Increasing levels of CO₂ in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005). In general, it is thought that calcifying invertebrates will be more sensitive to ocean acidification than non-calcifying invertebrates, which appear to have a more mixed response (Hofmann et al., 2010). Some hydroids are calcifying but the majority of hydroid species have exoskeletons made of chitin, including the two characterizing species of hydroid found within this biotope; Sertularia spp. and Hydrallmania falcata (Schmidt & Warner, 1991, Gehrmann, 2011). There is currently no available evidence on whether these two species are sensitive to a decrease in pH. However, one study found that a calcifying community of seagrass epiphytic cover was replaced by a community dominated by non-calcifying invertebrates including hydroids, tunicates and fleshy algae at natural CO₂ vents in areas of extremely low pH (pH 6.6 to 7.2), (Donnarumma & Lombardi, 2014). This observation  suggests many hydroids may be tolerant. In addition, seven epibiotic hydroid species were recorded at CO₂ vents in Italy, and three of these were abundant all year round, showing high tolerance to low pH levels (Gravili et al., 2021).

The prediction that calcifying species are always more sensitive than non-calcifying species is not always correct. A 0.3 unit decrease in pH to pH 7.8 (the decrease expected for the end of this century under the high emission scenario) led to no negative impacts on net calcification in the calcifying hydroid Millepora alcicornis (de Barros Marangoni et al., 2017). In comparison, the hydroid Obelia dichotoma with a chitinous exoskeleton (Mendoza-Becerril et al., 2017) experienced a significant reduction in settlement and growth of this species on settlement panels due to a 0.3 unit decrease to a pH of 7.8 (Brown et al., 2016). This corresponds to previous findings that early life history stages in invertebrates are known to be more susceptible to ocean acidification than adult stages (Hofmann et al., 2010). Widdicombe & Spicer (2008) suggested that the effects of pCO₂ and hypercapnia was likely to be species-specific rather than predictable by phylogeny or habitat.

Bryozoans are invertebrate calcifiers Therefore, they are potentially highly sensitive to ocean acidification (Smith, 2009). The decrease in water pH from global climate change could cause corrosion, changes in mineralogy and decrease the survival of bryozoans (Smith, 2014). In addition, elevated seawater temperatures generally increase the metabolic rate of bryozoans and is predicted to affect calcification (Smith, 2014; Moreno, 2020).

No direct evidence on the impacts of ocean acidification on the characterizing bryozoan species Flustra foliacea was found. Smith (2014) suggested that Flustra foliacea is poorly calcified but as a multi-branched bryozoan has the capacity to produce more carbonate over its long life. However, in a dissolution experiment conducted by Fortunato et al., (2013) on dead colonies, Flustra foliacea exhibited a faster loss of calcite over time compared other bryozoan colonies Hornera lichenoides and Cellaria sinuosa, which suggested it has a higher sensitivity to skeleton dissolution.

Swezey et al. (2017) observed that populations of bryozoans raised under high CO₂ (1254 μatm; pH 7.60) conditions grew faster, invested less in reproduction and produced lighter skeletons when compared to genetically identical clones raised under current surface atmospheric CO₂ values (400 μatm; pH 8.04). In addition, the bryozoans under high CO₂ altered the Mg/Ca ratio of skeletal calcite, which could be a protective mechanism against acidification (Swezey et al., 2017). Colonies of the bryozoan Myriapora truncate increased calcification rates and was able to survive an high CO₂ levels (mean pH 7.66) but calcification rates halted and mortality was observed in the species after a prolonged period of increased temperature (25 to 28°C) and extremely high CO₂ levels (mean pH 7.43) (Rodolofo – Metalpa et al., 2010). In addition, epibiotic bryozoan species were able to survive in low pH environment at a volcanic CO₂ and could tolerate pH fluctuations of more than 1 unit (Martin et al., 2008).

Sensitivity assessment. There is no species-specific evidence of the impact of ocean acidification on the characterizing species of this biotope. Under the middle emission scenario, it is unlikely that the characterizing species Flustra foliacea and Hydrallmania falcata will exhibit any negative effects to decrease of 0.15 pH units. Therefore, under the precautionary principle, resistance is assessed as ‘High’ and resilience is assessed as ‘High’. Therefore, this biotope is assessed as ‘Not sensitive’ under the middle emission scenario. Under the high emission scenario, there is a possibility that at least one of the characterizing species of this biotope (Flustra foliacea) could be affected by a reduction in pH. Therefore, resistance is assessed as ‘Medium’, and resilience as ‘Very Low’, as population loss would not recover due to the long-term nature of ocean acidification. Hence, this biotope is assessed as 'Medium' sensitivity under the high emission scenario but with ‘Low’ confidence.

Ocean acidification (middle)

Middle emission scenario benchmark: a further decrease in pH of 0.15 (annual mean) and corresponding 35% increase in H+ ions with no coastal aragonite undersaturation and the aragonite saturation horizon in the NE Atlantic, off the continental shelf, at a depth of 800 m by the end of this century 2081-2100. Further detail.

Evidence

Increasing levels of CO₂ in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005). In general, it is thought that calcifying invertebrates will be more sensitive to ocean acidification than non-calcifying invertebrates, which appear to have a more mixed response (Hofmann et al., 2010). Some hydroids are calcifying but the majority of hydroid species have exoskeletons made of chitin, including the two characterizing species of hydroid found within this biotope; Sertularia spp. and Hydrallmania falcata (Schmidt & Warner, 1991, Gehrmann, 2011). There is currently no available evidence on whether these two species are sensitive to a decrease in pH. However, one study found that a calcifying community of seagrass epiphytic cover was replaced by a community dominated by non-calcifying invertebrates including hydroids, tunicates and fleshy algae at natural CO₂ vents in areas of extremely low pH (pH 6.6 to 7.2), (Donnarumma & Lombardi, 2014). This observation  suggests many hydroids may be tolerant. In addition, seven epibiotic hydroid species were recorded at CO₂ vents in Italy, and three of these were abundant all year round, showing high tolerance to low pH levels (Gravili et al., 2021).

The prediction that calcifying species are always more sensitive than non-calcifying species is not always correct. A 0.3 unit decrease in pH to pH 7.8 (the decrease expected for the end of this century under the high emission scenario) led to no negative impacts on net calcification in the calcifying hydroid Millepora alcicornis (de Barros Marangoni et al., 2017). In comparison, the hydroid Obelia dichotoma with a chitinous exoskeleton (Mendoza-Becerril et al., 2017) experienced a significant reduction in settlement and growth of this species on settlement panels due to a 0.3 unit decrease to a pH of 7.8 (Brown et al., 2016). This corresponds to previous findings that early life history stages in invertebrates are known to be more susceptible to ocean acidification than adult stages (Hofmann et al., 2010). Widdicombe & Spicer (2008) suggested that the effects of pCO₂ and hypercapnia was likely to be species-specific rather than predictable by phylogeny or habitat.

Bryozoans are invertebrate calcifiers Therefore, they are potentially highly sensitive to ocean acidification (Smith, 2009). The decrease in water pH from global climate change could cause corrosion, changes in mineralogy and decrease the survival of bryozoans (Smith, 2014). In addition, elevated seawater temperatures generally increase the metabolic rate of bryozoans and is predicted to affect calcification (Smith, 2014; Moreno, 2020).

No direct evidence on the impacts of ocean acidification on the characterizing bryozoan species Flustra foliacea was found. Smith (2014) suggested that Flustra foliacea is poorly calcified but as a multi-branched bryozoan has the capacity to produce more carbonate over its long life. However, in a dissolution experiment conducted by Fortunato et al., (2013) on dead colonies, Flustra foliacea exhibited a faster loss of calcite over time compared other bryozoan colonies Hornera lichenoides and Cellaria sinuosa, which suggested it has a higher sensitivity to skeleton dissolution.

Swezey et al. (2017) observed that populations of bryozoans raised under high CO₂ (1254 μatm; pH 7.60) conditions grew faster, invested less in reproduction and produced lighter skeletons when compared to genetically identical clones raised under current surface atmospheric CO₂ values (400 μatm; pH 8.04). In addition, the bryozoans under high CO₂ altered the Mg/Ca ratio of skeletal calcite, which could be a protective mechanism against acidification (Swezey et al., 2017). Colonies of the bryozoan Myriapora truncate increased calcification rates and was able to survive an high CO₂ levels (mean pH 7.66) but calcification rates halted and mortality was observed in the species after a prolonged period of increased temperature (25 to 28°C) and extremely high CO₂ levels (mean pH 7.43) (Rodolofo – Metalpa et al., 2010). In addition, epibiotic bryozoan species were able to survive in low pH environment at a volcanic CO₂ and could tolerate pH fluctuations of more than 1 unit (Martin et al., 2008).

Sensitivity assessment. There is no species-specific evidence of the impact of ocean acidification on the characterizing species of this biotope. Under the middle emission scenario, it is unlikely that the characterizing species Flustra foliacea and Hydrallmania falcata will exhibit any negative effects to decrease of 0.15 pH units. Therefore, under the precautionary principle, resistance is assessed as ‘High’ and resilience is assessed as ‘High’. Therefore, this biotope is assessed as ‘Not sensitive’ under the middle emission scenario. Under the high emission scenario, there is a possibility that at least one of the characterizing species of this biotope (Flustra foliacea) could be affected by a reduction in pH. Therefore, resistance is assessed as ‘Medium’, and resilience as ‘Very Low’, as population loss would not recover due to the long-term nature of ocean acidification. Hence, this biotope is assessed as 'Medium' sensitivity under the high emission scenario but with ‘Low’ confidence.

Sea level rise (extreme)

Extreme scenario benchmark: a 107 cm rise in average UK by the end of this century (2018-2100). Further detail.

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm yr^-1 in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). The most recent projections on sea-level rise suggest a rise of 50 cm under the middle emission scenario, 70 cm under the high emission scenario, and 107 cm under the extreme scenario. Sertularia cupressina is known to reside at a depth of over 90 m in the Bay of Fundy, Canada (Henry & Kenchington, 2004), whilst Hydrallmania falcata can be found at depths of more than 100 m (www.obis.org), suggesting that, as long as the habitat (tide-swept sublittoral sand with cobbles or pebbles) remains the same these species will be tolerant of future sea-level rise for all three scenarios (mid-emission 50 cm, high emission 70 cm extreme scenario 107 cm).

Understanding of how sea-level rise will affect tidal energy, and the tide-swept nature of a habitat, is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. The effects of sea-level rise and increased wave action may be increased further due to storms and storm surges. IPCC (2019) note that the frequency of extreme sea-level events (e.g. due to storms) are predicted to increase as sea-level rises, however there is no consensus on the future storm and, hence, wave climate around UK coasts (Mossman et al., 2015; Lowe et al., 2018; Palmer et al., 2018).

Sensitivity assessment. This habitat occurs from 5 to 50 m and both Sertularia cupressina and Hydrallmania falcata are abundant at depths of up to 100 m, and Flustra foliacea abundant at depths up to 200 m. However, the habitat is structured by scour due to tidal streams and water flow. Any change to the habitat in terms of its tide-swept nature cannot be evaluated at the current time, although evidence suggests that changes to tidal currents and tidal amplitude in relation to sea-level rise will be site-specific. Therefore, under the available evidence, resistance to sea-level rise has been assessed as ‘High’ for both the middle (50 cm), and high (70 cm) emission scenario, and the extreme scenario (107 cm). As no recovery is deemed necessary, resilience has been assessed as ‘High’, and therefore this species has been classified as ‘Not sensitive’ to sea-level rise at each of the benchmarks albeit with ‘Low’ confidence. 

Sea level rise (high)

High emission scenario benchmark: a 70 cm rise in average UK by the end of this century (2018-2100). Further detail.

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm yr^-1 in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). The most recent projections on sea-level rise suggest a rise of 50 cm under the middle emission scenario, 70 cm under the high emission scenario, and 107 cm under the extreme scenario. Sertularia cupressina is known to reside at a depth of over 90 m in the Bay of Fundy, Canada (Henry & Kenchington, 2004), whilst Hydrallmania falcata can be found at depths of more than 100 m (www.obis.org), suggesting that, as long as the habitat (tide-swept sublittoral sand with cobbles or pebbles) remains the same these species will be tolerant of future sea-level rise for all three scenarios (mid-emission 50 cm, high emission 70 cm extreme scenario 107 cm).

Understanding of how sea-level rise will affect tidal energy, and the tide-swept nature of a habitat, is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. The effects of sea-level rise and increased wave action may be increased further due to storms and storm surges. IPCC (2019) note that the frequency of extreme sea-level events (e.g. due to storms) are predicted to increase as sea-level rises, however there is no consensus on the future storm and, hence, wave climate around UK coasts (Mossman et al., 2015; Lowe et al., 2018; Palmer et al., 2018).

Sensitivity assessment. This habitat occurs from 5 to 50 m and both Sertularia cupressina and Hydrallmania falcata are abundant at depths of up to 100 m, and Flustra foliacea abundant at depths up to 200 m. However, the habitat is structured by scour due to tidal streams and water flow. Any change to the habitat in terms of its tide-swept nature cannot be evaluated at the current time, although evidence suggests that changes to tidal currents and tidal amplitude in relation to sea-level rise will be site-specific. Therefore, under the available evidence, resistance to sea-level rise has been assessed as ‘High’ for both the middle (50 cm), and high (70 cm) emission scenario, and the extreme scenario (107 cm). As no recovery is deemed necessary, resilience has been assessed as ‘High’, and therefore this species has been classified as ‘Not sensitive’ to sea-level rise at each of the benchmarks albeit with ‘Low’ confidence. 

Sea level rise (middle)

Middle emission scenario benchmark: a 50 cm rise in average UK sea-level rise by the end of this century (2081-2100). Further detail.

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm yr^-1 in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). The most recent projections on sea-level rise suggest a rise of 50 cm under the middle emission scenario, 70 cm under the high emission scenario, and 107 cm under the extreme scenario. Sertularia cupressina is known to reside at a depth of over 90 m in the Bay of Fundy, Canada (Henry & Kenchington, 2004), whilst Hydrallmania falcata can be found at depths of more than 100 m (www.obis.org), suggesting that, as long as the habitat (tide-swept sublittoral sand with cobbles or pebbles) remains the same these species will be tolerant of future sea-level rise for all three scenarios (mid-emission 50 cm, high emission 70 cm extreme scenario 107 cm).

Understanding of how sea-level rise will affect tidal energy, and the tide-swept nature of a habitat, is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. The effects of sea-level rise and increased wave action may be increased further due to storms and storm surges. IPCC (2019) note that the frequency of extreme sea-level events (e.g. due to storms) are predicted to increase as sea-level rises, however there is no consensus on the future storm and, hence, wave climate around UK coasts (Mossman et al., 2015; Lowe et al., 2018; Palmer et al., 2018).

Sensitivity assessment. This habitat occurs from 5 to 50 m and both Sertularia cupressina and Hydrallmania falcata are abundant at depths of up to 100 m, and Flustra foliacea abundant at depths up to 200 m. However, the habitat is structured by scour due to tidal streams and water flow. Any change to the habitat in terms of its tide-swept nature cannot be evaluated at the current time, although evidence suggests that changes to tidal currents and tidal amplitude in relation to sea-level rise will be site-specific. Therefore, under the available evidence, resistance to sea-level rise has been assessed as ‘High’ for both the middle (50 cm), and high (70 cm) emission scenario, and the extreme scenario (107 cm). As no recovery is deemed necessary, resilience has been assessed as ‘High’, and therefore this species has been classified as ‘Not sensitive’ to sea-level rise at each of the benchmarks albeit with ‘Low’ confidence. 

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

Evidence

Gili & Hughes (1995) reported that temperature is a critical factor stimulating or preventing reproduction and that most species have an optimal temperature for reproduction. However, limited evidence for thermal thresholds and thermal ranges were available for the characterizing species recorded in this biotope.  Berrill (1949) reported that growth in Obelia commissularis (syn. dichotoma) was temperature dependent but ceased at 27°C.  Hydranths did not start to develop unless the temperature was less than 20°C and any hydranths under development would complete their development and rapidly regress at ca 25°C.  Berrill (1948) reported that Obelia species were absent from a buoy in July and August during excessively high summer temperatures in Booth Bay Harbour, Maine, USA.  Berrill (1948) reported that the abundance of Obelia species and other hydroids fluctuated greatly, disappearing and reappearing as temperatures rose and fell markedly above and below 20°C during this period.  The upwelling of cold water (8-10°C colder than surface water) allowed colonies of Obelia sp. to form in large numbers.  Cantero et al. (2002) describe the presence and year-round fertility of Obelia dichotoma, Kirchenpaureria pinnata, Nemertesia ramosa and Halecium spp.in the Mediterranean. 

Bugula spp. grows and reproduces in the summer months, however, day length and/or the phytoplankton bloom characteristic of temperate waters are probably more important than temperature (Ryland, 1967; 1970; Tyler-Walters, 2005c).  Bugula turbinata is a predominantly southern species in British waters (Lewis, 1964; Hayward & Ryland, 1998) but has been recorded as far north as Shetland (NBN, 2015). A long-term increase in temperature may increase its abundance in northern British waters and allow the species to extend its range. It occurs as far south as the Mediterranean (Rosso, 2003) and likely to tolerate increases in temperature, at the benchmark level.  Cocito & Sgorbini (2014) studied spatial and temporal patterns of colonial bryozoans in the Ligurian Sea over 9 years.  High temperature events were recorded, the first causing mass mortality among a number of species.  Alcyonidium diaphanum is commonly found across the British Isles and is probably widely distributed across North-West Europe (Fish  & Fish, 1996).

Sensitivity assessment. None of the characterizing species are at their southern distribution limit in the British Isles. No evidence for mortality linked to an increase in temperature in the British Isles was found.  The biotope is, therefore, assessed as having a resistance of ‘High’, a resilience of ‘High’ and is assessed as ‘Not sensitive’ at the benchmark level.

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

Evidence

Berrill (1949) reported that for Obelia, stolons grew, under optimal nutritive conditions, at less than 1 mm in 24 hrs at 10-12°C, 10 mm in 24 hrs at 16-17°C, and as much as 15-20 mm in 24 hrs at 20°C.  All important characterizing bryozoans (Alcyonidium diaphanum, Flustra foliacea, Bugula plumosa and Bugula flabellata) have been recorded across the British Isles, from the Channel Isles to the northern coast of Scotland (NBN, 2015). Alcyonium digitatum is recorded from Iceland in the north to Portugal in the south and it is unlikely that this species will be adversely affected by a long-term temperature change in British waters (Budd, 2008).  Alcyonium digitatum was also reported to be apparently unaffected by the severe winter of 1962-1963 (Crisp, 1964).  The hydroids Obelia dichotoma, Halecium Halecinum and Nemertesia sp. were recorded in Svalbard in the Arctic Circle (Orejas et al., 2012).

Sensitivity assessment. The majority of characterizing species occur in boreal environments, with none at their northerly distribution limit.  Therefore, resistance is likely to be ‘High’ with a resilience of ‘High’ and the biotope is probably ‘Not sensitive’ at the benchmark level.

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Studies on hydroids, in general, have found that prey capture rates may be affected by salinity and temperature (Gili & Hughes, 1995) although no evidence was found for species that characterize this biotope.  Soule & Soule (1979) cite Hastings (1927) who described the presence of five bryozoans in hypersaline conditions in the Suez Canal. ‘No evidence’ for mortality or tolerance of the characterizing bryozoans or hydroids in hypersaline conditions could be found.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

This biotope is recorded in full salinity habitats (Connor et al., 2004).  Little evidence for the characterizing hydroids could be found. Stebbing (1981a) found that, for the hydroid Campanularia flexuosa, growth was inhibited in 70% seawater (ca 25‰) and that exposure to below 30% seawater (ca 10‰) was lethal after 3 days. Ryland (1970) stated that, with a few exceptions, the Gymnolaemata bryozoans were fairly stenohaline and restricted to full salinity (30-35 ppt), noting that reduced salinities result in an impoverished bryozoan fauna. Flustra foliacea appears to be restricted to areas with high salinity (Tyler-Walters & Ballerstedt, 2007).  Dyrynda (1994) noted that Flustra foliacea and Alcyonidium diaphanum were probably restricted to the vicinity of the Poole Harbour entrance by their intolerance to reduced salinity. Although protected from extreme changes in salinity due to their subtidal habitat, severe hyposaline conditions could adversely affect Flustra foliacea colonies.

Sensitivity assessment. The characterizing bryozoans are likely to be affected by a reduction in salinity, and species diversity is likely to decrease.  Resistance is assessed as ‘Low’, resilience is assessed as ‘High’ and sensitivity is ‘Low’.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

Hayward & Ryland (1995b) noted that abundant communities of hydroids occur in narrow straits and headlands, which may experience high levels of water flow. Hydroids can bend passively with water flow to reduce drag forces to prevent detachment and enhance feeding (Gili & Hughes, 1995). The hydroid growth form also varies to adapt to prevailing conditions, allowing species to occur in a variety of habitats (Gili & Hughes, 1995). For example, Hiscock (1979b) assessed feeding behaviour of the hydroid Tubularia indivisa in response to different flow rates.  At flow rates <0.05 m/s, polyps actively moved tentacles.  Increasing the flow rate to 0.2 m/s increased capture rates but at higher flow rates from 0.5-0.9 m/s the tentacles were extended down current and pushed together and feeding efficiency was reduced. In general, flow rates are an important factor for feeding in hydroids and prey capture appears to be higher in more turbulent conditions that prevent self-shading by the colony (Gili & Hughes, 1995). The capture rate of zooplankton by hydroids is correlated with prey abundance (Gili & Hughes, 1995), thus prey availability can compensate for sub-optimal flow rates. Water movements are also important to hydroids to prevent siltation, which can cause death (Round et al., 1961). Tillin & Tyler-Walters (2014) suggested that the range of flow speeds experienced by biotopes in which hydroids are found indicate that a change (increase or decrease) in the maximum water flow experienced by mid-range populations for the short periods of peak spring tide flow would not have negative effects on this group. 

Water flow has been shown to be important for the development of bryozoan communities and the provision of suitable hard substrata for colonization (Eggleston, 1972b; Ryland, 1976). In addition, areas subject to the high mass transport of water such as the Menai Strait and tidal rapids generally support large numbers of bryozoan species (Moore, 1977a). Although, active suspension feeders, their feeding currents are probably fairly localized and they are dependent on water flow to bring adequate food supplies within reach (McKinney, 1986). A substantial decrease in water flow will probably result in impaired growth due to a reduction in food availability, and an increased risk of siltation (Tyler-Walters, 2005c). Okamura (1984) reported that an increase in water flow from slow flow (1-2 cm/s) to fast flow (10-12 cm/s) reduced feeding efficiency in small colonies but not in large colonies of Bugula stolonifera. 

Flustra foliacea colonies are flexible, robust and reach high abundances in areas subject to strong currents and tidal streams (Stebbing, 1971a; Eggleston, 1972b; Knight-Jones & Nelson-Smith, 1977; Hiscock, 1983, 1985; Holme & Wilson, 1985). Dyrynda (1994) suggested that mature fronded colonies do not occur on unstable substratum due to the drag caused by their fronds, resulting in rafting of colonies on shells or the rolling of pebbles and cobbles, resulting in the destruction of the colony. Dyrynda (1994) reported that the distribution of Flustra foliacea in the current swept entrance to Poole Harbour was restricted to circalittoral boulders, on which it dominated as nearly mono-specific stands. While, the pumping activity of the lophophores provide the greatest proportion of the colonies food requirements (Hayward & Ryland, 1998), the current generated is probably very localized and the colonies are likely to be dependant on water currents for food supply.  A significant decrease in water flow is likely to result in a decrease in the abundance of bryozoans. 

Sensitivity assessment. The biotope experiences moderate tidal streams and substantial increase or decrease would probably result in a decline of the biotope.  However, a 0.1 – 0.2 m/s change (the benchmark) is unlikely to significantly impact the characterizing species.  Resistance is, therefore, assessed as ‘High’, resilience is assessed as ‘High’ and the biotope is assessed as ‘Not Sensitive’ at the benchmark level. 

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year or 2) an increase in relative sea level or decrease in high water level for ≥1 year. Further detail

Evidence

Changes in emergence are not relevant to this biotope as it is restricted to fully subtidal/circalittoral conditions-The pressure benchmark is relevant only to littoral and shallow sublittoral fringe biotopes.

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

Jackson (2004) reported that Nemertesia ramosa was intolerant of high wave exposure and only found in sheltered areas.  Faucci et al. (2000) recorded hydroid communities at two sites of different wave exposure and recorded the presence of Obelia dichotoma and Halecium spp. in both the exposed and sheltered sites, but only found Kirchenpaueria sp. in the sheltered site. Bugula spp. produce flexible erect tufts, which are likely to move with the oscillatory flow created by wave action. Bugula turbinata has been recorded from very wave exposed to very wave sheltered habitats (Tyler-Walters, 2005c). Flustra foliacea occurs from very wave exposed to sheltered waters, although probably limited to deeper waters in very wave exposed conditions (Tyler-Walters & Ballerstedt, 2007). The oscillatory water flow generated by wave action may be more damaging than constant strong currents, e.g. strong wave action may generate an oscillatory flow of 2 m/sec at 20 m (Hiscock, 1983, 1985). Flustra foliacea is a common member of the flotsam, having been removed from its substratum by storms. Whilst the biotope is circalittoral, a severe increase in wave exposure (e.g. storms) could affect bryozoans colonies, especially on mobile substrata such as cobbles and pebbles.  Cocito et al. (1998) described a severe winter storm of 1993 had devastating effects on the same Flustra foliacea population, sweeping away most of the colonies down to 11 m.

Sensitivity assessment. A significant increase in wave exposure could affect the characterizing species due to increased scour and movement of mobile substrata.  A significant decrease may also affect the biotope.  However, a change at the benchmark level would be unlikely to affect the characterizing species.  Resistance is, therefore, assessed as ‘High’, resilience is assessed as ‘High’ and the biotope is assessed as ‘Not sensitive’ at the benchmark level. 

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

Although no information on the effects of heavy metals on the assessed hydroids was found, evidence suggests that hydroids may suffer at least sub-lethal effects and possibly morphological changes and reduced growth due to heavy metal contamination. Various heavy metals have been shown to have sublethal effects on growth in the few hydroids studied experimentally (Bryan, 1984). Stebbing (1981a) reported that Cu, Cd, and tributyl tin fluoride affected growth regulators in Laomedea (as Campanularia) flexuosa resulting in increased growth. Stebbing (1976) reported that 1 µg/l Hg²⁺ was stimulatory, although the effect was transitory, exposure resulting in reduced growth towards the end of his 11-day experiments. Cadmium (Cd) was reported to cause irreversible retraction of 50% of hydranths in Laomedea loveni after 7 days exposure at concentrations between 3 µg/l (at 17.5°C and 10 ppt salinity) and 80 µg/l (at 7.5°C and 25 ppt salinity) (Theede et al., 1979). Laomedea loveni was more tolerant of Cd exposure at low temperatures and low salinities. Karbe (1972, summary only) examined the effects of heavy metals on the hydroid Eirene viridula (Campanulidae). He noted that Cd and Hg caused cumulative effects, and morphological changes. Mercury (Hg) caused irreversible damage at concentrations as low as 0.02 ppm. He reported threshold levels of heavy metals for acute effects in Eirene viridula of 1.5-3 ppm Zn, 1-3 ppm Pb, 0.1-0.3 ppm Cd, 0.03-0.06 ppm Cu and 0.001-0.003 ppm Hg. Karbe (1972, summary only) suggested that Eirene viridula was a sensitive test organism when compared to other organisms. Although no information on the effects of heavy metals on assessed hydroid species was found, the above evidence suggests that hydroids may suffer at least sub-lethal effects and possibly morphological changes and reduced growth due to heavy metal contamination.

Bryozoans are common members of fouling communities and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints. Bryozoans were also shown to bioaccumulate heavy metals to a certain extent (Soule & Soule, 1979; Holt et al., 1995).

Nevertheless, this pressure is Not assessed but evidence is presented where available.

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available.

Filter feeders are highly sensitive to oil pollution, particularly those inhabiting the tidal zones which experience high exposure and show correspondingly high mortality, as are bottom dwelling organisms in areas where oil components are deposited by sedimentation (Zahn et al., 1981). Oil pollution is mainly a surface phenomenon its impact upon circalittoral turf communities is likely to be limited. However, as in the case of the Prestige oil spill off the coast of France, high swell and winds can cause oil pollutants to mix with the seawater and potentially negatively affect sub-littoral habitats (Castège et al., 2014). Banks & Brown (2002) found that exposure to crude oil significantly impacted recruitment in the bryozoan Membranipora savartii. Tethya lyncurium concentrated BaP (benzo[a ]pyrene) to 40 times the external concentration and no significant repair of DNA was observed in the sponges, which, in higher animals, would likely lead to cancers. As sponge cells are not organized into organs the long-term effects are uncertain (Zahn et al., 1981).

Little information on the effects of hydrocarbons on bryozoans was found. Ryland & de Putron (1998) did not detect adverse effects of oil contamination on the bryozoan Alcyonidium spp. or other sessile fauna in Milford Haven or St. Catherine's Island, south Pembrokeshire. Houghton et al. (1996) reported a reduction in the abundance of intertidal encrusting bryozoa (no species given) at oiled sites after the Exxon Valdez oil spill. Soule & Soule (1979) reported that the encrusting bryozoan Membranipora villosa was not found in the impacted area for 7 months after the December 1976 Bunker C oil spill in Los Angeles Harbour. Additionally, Soule & Soule (1979) reported that Bugula neritina was lost from breakwater rocks in the vicinity (in December 1979) of the Bunker C oil spill and had not recovered within a year. However, Bugula neritina had returned to a nearby area within 5 months (May 1977) even though the area was still affected by sheens of oil. Furthermore, only three of eight recorded species two weeks after the incident were present in April within the affected breakwater area. By June all the species had been replaced by dense growths of the erect bryozoan Scrupocellaria diegensis.

Mohammad (1974) reported that Bugula spp. and Membranipora spp. were excluded from settlement panels near an oil terminal in Kuwait subject to minor but frequent oil spills. Encrusting bryozoans are also probably intolerant of the smothering effects of acute hydrocarbon contamination and pollution, resulting in suffocation of colonies and communities may be lost or damaged. Circalittoral communities are likely to be protected from the direct effects of oil spills by their depth. However, the biotope may be exposed to emulsified oil treated with dispersants, especially in areas of turbulence, or may be exposed to water soluble fractions of oils, PAHs or oil adsorbed onto particulates (Tyler-Walters, 2002).

Little information of the effects of hydrocarbons on hydroids was found although hydroid species adapted to a wide variation in environmental factors and with cosmopolitan distributions tend to be more tolerant of polluted waters (Boero, 1984; Gili & Hughes, 1995).

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available.

The species richness of hydroid communities decreases with increasing pollution but hydroid species adapted to a wide variation in environmental factors and with cosmopolitan distributions tend to be more tolerant of polluted waters (Boero, 1984; Gili & Hughes, 1995). Stebbing (1981a) cited reports of growth stimulation in Obelia geniculata caused by methyl cholanthrene and dibenzanthrene. Hoare & Hiscock (1974) suggested that the Bryozoa (as Polyzoa) were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey, e.g. Electra pilosa occurred at lower abundance on laminarian holdfasts within the bay, compared to sites outside the affected area.

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

'No evidence' was found.

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status). Further detail

Evidence

In general, respiration in most marine invertebrates does not appear to be significantly affected until extremely low concentrations are reached. For many benthic invertebrates, this concentration is about 2 ml/l (Herreid, 1980; Rosenberg et al., 1991; Diaz & Rosenberg, 1995). Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2mg/l.

Hydroids mainly inhabit environments in which the oxygen concentration exceeds 5 ml/l (Gili & Hughes, 1995). Although no information was found on oxygen consumption for the characterizing hydroids, Sagasti et al. (2000) reported that epifaunal species, including several hydroids and bryozoans in the York River, Chesapeake Bay, tolerated summer hypoxic episodes of between 0.5 and 2 mg O₂/l (0.36 and 1.4 ml/l) for 5-7 days at a time, with few changes in abundance or species composition, although bryozoans were more abundant in the area with generally higher oxygen. However, estuarine species are likely to be better adapted to periodic changes in oxygenation. 

Sensitivity assessment. Whilst hydroids and bryozoans have been shown to tolerate short anoxic events (Sagasti et al., 2000) and the sand scoured nature of the biotope would likely result in occasional burial, an event at the benchmark level would likely result in some mortality.  Resistance is, therefore, assessed as ‘Medium’, resilience as ‘High’ and sensitivity as ‘Low’.

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

Evidence

Witt et al. (2004) found that the hydroid Obelia spp. was more abundant in a sewage disposal area in the Weser estuary (Germany) which experienced sedimentation of 1 cm for more than 25 days.  It should be noted that another hydroid (Sertularia cupressina) was reduced in abundance when compared with unimpacted reference areas.   As suspension feeders, an increase in organic content at the benchmark is likely to be of benefit to the characterizing hydroids. Hartikainen et al. (2009) reported that increased nutrient concentrations resulted in freshwater bryozoans achieving higher biomass.  O’Dea & Okamura (2000) found that annual growth of Flustra foliacea in western Europe has substantially increased since 1970.  They suggest that this could be due to eutrophication in coastal regions due to organic pollution, leading to increased phytoplankton biomass (see Allen et al., 1998).

However, this biotope is considered to be 'Not sensitive' at the pressure benchmark, that assumes compliance with good status as defined by the WFD.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr. Further detail

Evidence

Witt et al. (2004) found that the hydroid Obelia spp. was more abundant in a sewage disposal area in the Weser estuary (Germany) which experienced sedimentation of 1 cm for more than 25 days.  It should be noted that another hydroid (Sertularia cupressina) was reduced in abundance when compared with unimpacted reference areas.   As suspension feeders, an increase in organic content at the benchmark is likely to be of benefit to the characterizing hydroids. O’Dea & Okamura (2000) found that annual growth of Flustra foliacea in western Europe has substantially increased since 1970.  They suggest that this could be due to eutrophication in coastal regions due to organic pollution, leading to increased phytoplankton biomass (see Allen et al., 1998).  Mayer-Pinto & Junqueira (2003) studies the effects of organic pollution on fouling communities in Brazil and found that some tolerance of polluted/unpolluted artificial reefs varied among bryozoan species.  It should be noted that Bugula spp. preferred the polluted sites.

Sensitivity assessment. Whilst an increase in organic matter would likely be removed relatively rapidly by water movement in this biotope, such an increase would likely be beneficial to the characterizing species.  Resistance is therefore assessed as ‘High’, resilience as ‘High’ and the biotope is probably ‘Not sensitive’ at the benchmark level.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

Evidence

All marine habitats and benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very low’). Sensitivity within the direct spatial footprint of this pressure is, therefore ‘High’. Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata or vice-versa. Further detail

Evidence

This biotope is characterized by the hard substratum provided by the pebbles and cobbles to which the key characterizing species can firmly attach to (Connor et al., 2004). A change to a mobile gravel or soft sedimentary substratum would significantly alter the character of the biotope. The biotope is considered to have 'No' resistance to this pressure based on a change to a soft sediment substratum, recovery of the biological assemblage (following habitat restoration) is considered to be 'Medium'. However, the pressure benchmark is considered to refer to a permanent change and recovery is, therefore ‘Very low’.  Sensitivity is, therefore, assessed as 'High'.

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

CR.HCR.XFa.SpNemAdi is characterized by the hard substratum provided by the pebbles and cobbles. SS.SMx.CMx.FluHyd and SS.SSa.IFiSa.ScupHyd are dominated by hard substrata on sediment (Connor et al., 2004). A change to a mobile gravel or soft sedimentary substratum would significantly alter the character of the biotope. The biotope is considered to have a resistance of 'None' to this pressure based on a change to a soft sediment substratum, recovery of the biological assemblage (following habitat restoration) is considered to be 'High'. However, the pressure benchmark is considered to refer to a permanent change and recovery is, therefore ‘Very low’.  Sensitivity is, therefore, assessed as 'High'.

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock). Further detail

Evidence

The species characterizing these biotopes are epifauna occurring on the cobbles and pebbles that characterize this biotope (Connor et al., 2004).  Removal of the substratum would remove both the habitat (boulders, cobbles and pebbles) and the characterizing, attached species.

Sensitivity assessment. Biotope resistance is assessed as ‘None’ (in the extraction footprint), resilience (following habitat restoration, or where the underlying substratum remains the same) is assessed as ‘Medium’. Sensitivity is, therefore, assessed as ‘Medium’. Recovery will be prolonged (and sensitivity greater) where the entire habitat is removed and restoration (artificial or natural) to the previous state does not occur.

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

The species characterizing this biotope occur on the rock surface and therefore have no protection from surface abrasion. High levels of abrasion from scouring by mobile sands and gravels is an important structuring factor in this biotope (Connor et al., 2004) and may prevent succession. Where individuals are attached to mobile pebbles, cobbles and boulders rather than bedrock, surfaces can be displaced and turned over preventing feeding and leading to smothering. Physical disturbance by fishing gear has been shown to adversely affect emergent epifaunal communities with hydroid and bryozoan matrices reported to be greatly reduced in fished areas (Jennings & Kaiser, 1998). Heavy mobile gears could also result in movement of boulders (Bullimore, 1985; Jennings & Kaiser, 1998).   

The available evidence indicates that hydroids can be entangled and removed by abrasion.  Drop down video surveys of Scottish reefs exposed to trawling showed that visual evidence of damage to bryozoans and hydroids on rock surfaces was generally limited and restricted to scrape scars on boulders (Boulcott & Howell, 2011).  The study showed that damage is incremental with damage increasing with the frequency of trawls rather than a blanket effect occurring on the pass of the first trawls. The results indicated that epifaunal species, including the sponge Pachymatisma johnstoni, were highly damaged by the experimental trawl. Note Boulcott & Howell (2011) did not mention the abrasion caused by fully loaded collection bags on the Newhaven dredges. A fully loaded Newhaven dredge may cause higher damage to the community as indicated in their study.

Re-sampling of grounds that were historically studied (from the 1930s) indicates that some species have increased in areas subject to scallop fishing (Bradshaw et al., 2002).  This study also found a (unquantified) increase in abundance of tough stemmed hydroids including Nemertesia spp.. Its morphology may have prevented excessive damage.  Bradshaw et al. (2002) suggested that as well as having high resistance to abrasion pressures, Nemertesia spp. have benthic larvae that could rapidly colonize disturbed areas with newly exposed substrata close to the adult. Hydroids may also recover rapidly as the surface covering of hydrorhizae may remain largely intact, from which new uprights are likely to grow. In addition, the resultant fragments of colonies may be able to develop into new colonies. Hydroid colonies were still present in the heavily fished area, albeit at lower densities than in the closed area. This may largely be because the Isle of Man scallop fishery is closed from 1^st June to 31^st October (Andrews et al., 2011), so at the time the samples were taken for the study in question, the seabed had been undredged for at least 3.5 months (Bradshaw et al., 2003). The summer period is also the peak growing/breeding season for many marine species.

Sensitivity assessment. Given the sessile, erect nature of the hydroids and bryozoans, damage and mortality following a physical disturbance effect are likely to be significant, however, some studies have brought into question the extent of damage to the faunal turf. Abrasion from scouring by sand, mobile cobbles and pebbles is an important structuring factor in this biotope (Connor et al., 2004) and the persistence of the assemblage may depend on rapid recovery together with scour resistance (e.g. Flustra).  Therefore, resistance is assessed as ‘Low’, resilience as ‘Medium’, and sensitivity is assessed as ‘Medium’.

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

This biotope is characterized by mobile pebbles and cobbles, pressures that lead to penetration and disturbance could damage associated species through abrasion and by overturning surfaces could result in the smothering of fauna or reductions in respiration, feeding efficiency or fertilization of gametes in the water column. The biotope is, however, likely to be exposed to at least seasonal movement of substrata and this movement and scour maintains this biotope by preventing species that require more stable habitats from colonizing and developing stable populations (Connor et al., 2004). Evidence presented above for surface abrasion is considered equally relevant to this pressure as abrasion in this biotope is likely to lead to movement and displacement of mobile substrata.

Sensitivity assessment. The impact of pressures that disturb and penetrate the mobile substrata will depend on the footprint, duration and magnitude of the pressure. Abrasion from scouring by sand, mobile cobbles and pebbles is an important structuring factor in this biotope (Connor et al., 2004) and the persistence of the assemblage may depend on rapid recovery together with scour resistance (e.g. Flustra).  Therefore, resistance is assessed as ‘Low’, resilience as ‘Medium’, and sensitivity is assessed as ‘Medium’.

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate for one year. Further detail

Evidence

An increase in suspended sediment may have a deleterious effect on the suspension feeding community. It is likely to clog their feeding apparatus to some degree, resulting in a reduced ingestion over the benchmark period and, subsequently, a decrease in growth rate (Jackson, 2004). As the hydroids capture small prey in suspension (Gili & Hughes, 1995), a reduction in feeding efficiency could potentially lead to a reduction in overall biomass. Nemertesia ramosa is a passive suspension feeder, extracting seston from the water column. Increased siltation may clog up the feeding apparatus, requiring energetic expenditure to clear. However, recovery is likely to take only a few days (Jackson, 2004).

Bryozoans are suspension feeders that may be adversely affected by increases in suspended sediment, due to clogging of their feeding apparatus.  However, Tyler-Walters & Ballerstedt (2007) reported Flustra foliacea as tolerant to increased suspended sediment based on its occurrence in areas of high suspended sediment e.g. abundant in turbid, fast flowing waters of the Menai Straits (Moore 1977a). Also, communities dominated by Flustra foliacea were described on tide swept seabed, exposed to high levels of suspended sediment and sediment scour in the English Channel (Holme & Wilson, 1985). Flustra foliacea is also characteristic of sediment scoured, silty rock communities CR.HCR.XFa.FluCoAs and CR.MCR.EcCr.UrtScr (Connor et al., 2004). 

Sensitivity assessment. Whilst an increase in suspended sediment may result in extra energetic expenditure in cleaning, it is unlikely to increase mortality. Therefore, resistance has been assessed as ‘High’, resilience as ‘High’ and the biotope is ‘Not Sensitive’ at the benchmark level. 

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

In general, it appears that hydroids are sensitive to silting (Boero, 1984; Gili & Hughes, 1995) and the decline in beds in the Wadden Sea have been linked to environmental changes including siltation.  Round et al. (1961) reported that the hydroid Sertularia (now Amphisbetia) operculata died when covered with a layer of silt after being transplanted to sheltered conditions.  Boero (1984) suggested that deep water hydroid species develop upright, thin colonies that accumulate little sediment, while species in turbulent water movement were adequately cleaned of silt by water movement.

Hughes (1977) found that maturing hydroids that had been smothered with detritus and silt lost most of the hydrocladia and hydranths. After one month, the hydroids were seen to have recovered, but although neither the growth rate nor the reproductive potential appeared to have been affected, the viability of the planulae may have been affected.  Nemertesia ramosa is an upright hydroid with a height of up to 15 cm. The colony structure is fairly tough and flexible. Smothering with 5 cm of sediment may cover over some individuals, while others may just have the lower section of the main stem covered (Hayward & Ryland, 1994).  Obelia dichotoma stems grow to 5 cm, while polysiphonic structures can reach up to 35 cm in height.  Halecium halecinum can grow up to 25 cm and Kirchenpaueria pinnata can grow to ca 10 cm (Hayward & Ryland, 1994).  Some of the community is therefore likely to survive smothering by 5 cm.

Smothering by 5 cm of sediment is likely to prevent feeding, and hence growth and reproduction, as well as respiration in the bryozoans. In addition, associated sediment abrasion may remove the bryozoan colonies. A layer of sediment will probably also interfere with larval settlement (Tyler-Walters, 2005c). However, Flustra foliacea dominated communities were subject to sediment transport (mainly sand) and periodic, temporary, burial (ca <5 cm) n a tide-swept region of the central English Channel (Holme & Wilson, 1985).

Sensitivity assessment. Whilst 5 cm of deposition may bury some of the characterizing species, the biotope experiences moderate water flow and sediment is likely to be removed rapidly.  The biotope is sand scoured and occasional disposition events are likely to occur.  Therefore, resistance is assessed as ‘High’, resilience as ‘High’ and the biotope is assessed as ‘Not sensitive’ at the benchmark level.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

In general, it appears that hydroids are sensitive to silting (Boero, 1984; Gili & Hughes, 1995) and the decline in beds in the Wadden Sea have been linked to environmental changes including siltation. Round et al. (1961) reported that the hydroid Sertularia (now Amphisbetia) operculata died when covered with a layer of silt after being transplanted to sheltered conditions. Boero (1984) suggested that deep water hydroid species develop upright, thin colonies that accumulate little sediment, while species in turbulent water movement were adequately cleaned of silt by water movement.

Hughes (1977) found that maturing hydroids that had been smothered with detritus and silt lost most of the hydrocladia and hydranths. After one month, the hydroids were seen to have recovered, but although neither the growth rate nor the reproductive potential appeared to have been affected, the viability of the planulae may have been affected.  Nemertesia ramosa is an upright hydroid with a height of up to 15 cm. The colony structure is fairly tough and flexible. Smothering with 5 cm of sediment may cover over some individuals, while others may just have the lower section of the main stem covered (Hayward & Ryland, 1994).  Obelia dichotoma stems grow to 5 cm, while polysiphonic structures can reach up to 35 cm in height.  Halecium halecinum can grow up to 25 cm and Kirchenpaueria pinnata can grow to ca 10 cm (Hayward & Ryland, 1994).  Some of the community is, therefore, likely to survive smothering by 5 cm.

Smothering by 30 cm of sediment is likely to prevent feeding, and hence growth and reproduction, as well as respiration in the bryozoans. In addition, associated sediment abrasion may remove the bryozoan colonies. A layer of sediment will probably also interfere with larval settlement (Tyler-Walters, 2005c). However, Flustra foliacea dominated communities were subject to sediment transport (mainly sand) and periodic, temporary, burial (ca <5 cm) n a tide-swept region of the central English Channel (Holme & Wilson, 1985).

Sensitivity assessment. The biotope occurs in sand scoured areas exposed to moderate water movement and deposited sediment would eventually be removed.  However, 30 cm of sediment would bury almost all characterizing species except for those on large boulders and would result in some mortality. Resistance is, therefore, assessed as ‘Medium’, resilience as ‘High’ and sensitivity as ‘Low’.

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline). Further detail

Evidence

Not assessed.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

Evidence

‘No evidence’ was found.

Underwater noise changes

Benchmark. MSFD indicator levels (SEL or peak SPL) exceeded for 20% of days in a calendar year. Further detail

Evidence

Stanley et al. (2014) studied the effects of vessel noise on fouling communities and found that the bryozoans Bugula neritina, Watersipora arcuate and Watersipora subtorquata responded positively.  More than twice as many bryozoans settled and established on surfaces with vessel noise (128 dB in the 30–10,000 Hz range) compared to those in silent conditions.  Growth was also significantly higher in bryozoans exposed to noise, with 20% higher growth rate in encrusting and 35% higher growth rate in branching species. No evidence could be found for the effects of noise or vibrations on the characterizing hydroids or sponges could be found.  The characterizing species are unlikely to be negatively affected by noise and resistance is, therefore, assessed as ‘High’, resilience as ‘High’ and sensitivity as ‘Not Sensitive’.

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

Gili & Hughes (1995) reviewed the effect of light on a number of hydroids and found that there is a general tendency for most hydroids to be less abundant in well-lit situations, potentially due to increased competition with macroalgae.  Whilst hydroid larvae can be positively or negatively phototactic, the planulae of Nemertesia antennina show no response to light (Hughes, 1977).

Jones et al. (2012) compiled a report on the monitoring of sponges around Skomer Island and found that many sponges, particularly encrusting species, preferred vertical or shaded bedrock to open, light surfaces.   Flustra foliacea larvae are positively phototactic on release, swimming for only short periods (Hayward & Ryland, 1998), however, at the depths Flustra foliacea can occur, light may not be important.

Sensitivity assessment: Whilst sponges seem to favour shaded areas in which to settle, it is unlikely that changes at the benchmark pressure would result in mortality.  Resistance to this pressure is assessed as 'High' and resilience as 'High'. This biotope is, therefore, assessed as 'Not sensitive'. 

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion. Further detail

Evidence

Barriers and changes in tidal excursion are 'Not relevant' to biotopes restricted to open waters.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

Evidence

'Not relevant' to seabed habitats.  NB. Collision by grounding vessels is addressed under ‘surface abrasion’.

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

Evidence

'Not relevant'.

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

Bugula spp. are classed as fouling bryozoans, and may be found in the intake pipes of ships or power stations, and on ships hulls. The geographic distribution of Bugula spp. has been extended by transportation by shipping (Ryland, 1967). However, no information on transportation of Bugula turbinata was found.  Therefore, there was ‘No evidence’ on which to assess this pressure.

Introduction or spread of invasive non-indigenous species

Benchmark. The introduction of one or more invasive non-indigenous species (INIS). Further detail

Evidence

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and the Essex coast in 1887-1890. It has spread through expansion and introductions along the full extent of the English Channel and into the European mainland (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Helmer et al., 2019; Hinz et al., 2011; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015).

Crepidula fornicata is recorded from shallow, sheltered bays, lagoons and estuaries or the sheltered sides of islands, in variable salinity (18 to 40) although it prefers ca 30 (Tillin et al., 2020). Larvae require hard substrata for settlement. It prefers muddy gravelly, shell-rich, substrata that include gravel, or shells of other Crepidula, or other species e.g., oysters, and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. But it also recorded in a wide variety of habitats including clean sands, artificial substrata, Sabellaria alveolata reefs and areas subject to moderately strong tidal streams (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015; Tillin et al., 2020). 

High densities of Crepidula fornicata cause ecological impacts on sedimentary habitats. The species can form dense carpets that can smother the seabed in shallow bays, changing and modifying the habitat structure. At high densities, the species physically smothers the sediment, and the resultant build-up of silt, pseudofaeces, and faeces is deposited and trapped within the bed (Tillin et al., 2020, Fitzgerald, 2007, Blanchard, 2009, Stiger-Pouvreau & Thouzeau, 2015). The biodeposition rates of Crepidula are extremely high and once deposited, form an anoxic mud, making the environment suitable for other species, including most infauna (Stiger-Pouvreau & Thouzeau, 2015, Blanchard, 2009). For example, in fine sands, the community is replaced by a reef of slipper limpets, that provide hard substrata for sessile suspension-feeders (e.g., sea squirts, tube worms and fixed shellfish), while mobile carnivorous microfauna occupy species between or within shells, resulting in a homogeneous Crepidula dominated habitat (Blanchard, 2009). Blanchard (2009) suggested the transition occurred and became irreversible at 50% cover of the limpet. De Montaudouin et al. (2018) suggested that homogenization occurred above a threshold of 20-50 Crepidula /m². 

Impacts on the structure of benthic communities will depend on the type of habitat that Crepidula colonizes. De Montaudouin & Sauriau (1999) reported that in muddy sediment dominated by deposit-feeders, species richness, abundance and biomass increased in the presence of high densities of Crepidula (ca 562 to 4772 ind./m²), in the Bay of Marennes-Oléron, presumably because the Crepidula bed provided hard substrata in an otherwise sedimentary habitat. In medium sands, Crepidula density was moderate (330-1300 ind./m²) but there was no significant difference between communities in the presence of Crepidula. Intertidal coarse sediment was less suitable for Crepidula with only moderate or low abundances (11 ind./m²) and its presence did not affect the abundance or diversity of macrofauna. However, there was a higher abundance of suspension–feeders and mobile Crustacea in the absence of Crepidula (De Montaudouin & Sauriau, 1999). The presence of Crepidula as an ecosystem engineer has created a range of new niche habitats, reducing biodiversity as it modifies habitats (Fitzgerald, 2007). De Montaudouin et al. (1999) concluded that Crepidula did not influence macroinvertebrate diversity or density significantly under experimental conditions, on fine sands in Arcachon Bay, France. De Montaudouin et al. (2018) noted that the limpet reef increased the species diversity in the bed, but homogenised diversity compared to areas where the limpets were absent. In the Milford Haven Waterway (MHW), the highest densities of Crepidula were found in areas of sediment with hard substrata, e.g., mixed fine sediment with shell or gravel or both (grain sizes 16-256 mm) but, while Crepidula density increased as gravel cover increased in the subtidal, the reverse was found in the intertidal (Bohn et al., 2015). Bohn et al. (2015) suggested that high densities of Crepidula in high-energy environments were possible in the subtidal but not the intertidal, suggesting the availability of this substratum type is beneficial for its establishment. Hinz et al. (2011) reported a substantial increase in the occurrence of Crepidula off the Isle of Wight, between 1958 and 2006, at a depth of ca 60 m, on hard substrata (gravel, cobbles, and boulders), swept by strong tidal streams. Presumably, Crepidula is more tolerant of tidal flow than the oscillatory flow caused by wave action which may be less suitable (Tillin et al., 2020). 

Sensitivity assessment. The above evidence suggests that Crepidula could colonize mixed sediment habitats in the subtidal, typical of this biotope, due to the presence of gravel, shells, cobbles, or any other hard substrata that can be used for larvae settlement (Tillin et al., 2020). Therefore, Crepidula has the potential to colonize, and modify the habitat and its associated community due to the introduction of Crepidula shell biomass, silt, pseudofaeces and faeces (Blanchard, 2009; Tillin et al., 2020), as occurs in maerl gravels (Grall & Hall-Spencer, 2003) resulting in the loss of the biotope. This habitat is exposed to moderately exposed, in which wave action and storms may mobilise the sediment (JNCC, 2022), which may mitigate or prevent colonization by Crepidula at high densities, although Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011).

A number of invasive bryozoans are of concern including Schizoporella japonica (Ryland et al., 2014) and Tricellaria inopinata (Dyrynda et al., 2000; Cook et al., 2013b). The high levels of scour in this biotope will limit the establishment of all but the most scour-resistant invasive non-indigenous species (INIS) from this biotope and no direct evidence was found for the effects of INIS on this biotope. There are records of Crepidula occurring in this biotope in the Solent (Hinz et al., 2011), but due to wave action and storms, the scour from cobbles and pebbles might hinder colonization. Therefore, the habitat may be more suitable for Crepidula where water movement is meditated by tidal flow rather than wave action, e.g., the deeper examples of the biotope, but Crepidula might not reach high densities.

Hence, resistance is assessed as 'Medium', due to the biotopes' exposure to wave action and possibly storms. Resilience is assessed as 'Very low', as it would require the removal of Crepidula, probably by artificial means. Hence, the biotope sensitivity is assessed as 'Medium' based on the worst-case scenario. Further evidence is required on the presence of Crepidula within this biotope, so the confidence in the assessment is 'Low'.

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus). Further detail

Evidence

Hydroids exhibit astonishing regeneration and rapid recovery from injury (Sparks, 1972) and the only inflammatory response is active phagocytosis (Tokin & Yaricheva, 1959;1961, as cited in Sparks, 1972).  No record of diseases in the characterizing hydroids could be found. No evidence for disease in the characterizing bryozoans could be found.

Sensitivity assessment. Sponge diseases have caused limited mortality in some species in the British Isles, although mass mortality and even extinction have been reported further afield.  Whilst research is on-going into sponge disease in the UK, there is ‘No evidence’ of mass mortality in the important characterizing species.

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

Despite historic harvesting of the hydroid Sertularia cupressina in the Wadden Sea (Wagler et al., 2009), no evidence for the harvesting of the characterizing hydroids could be found in the UK and targeted extraction is highly unlikely.  No evidence for commercial exploitation of bryozoans could be found.  Should removal of target species occur, the sessile, epifaunal nature of the characterizing species would result in little resistance to this pressure.

Sensitivity assessment. The characterizing species are sessile epifauna and would have no resistance to targeted extraction.  Based on the above observations, resistance is assessed as ‘None’ and resilience as ‘Medium’ with a resultant sensitivity of ‘Medium’.

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

This biotope may be removed or damaged by static or mobile gears that are targeting other species. These direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures. The sensitivity assessment for this pressure considers any biological/ecological effects resulting from the removal of non-target species on this biotope. However, incidental removal of the characteristic epifauna due to by-catch is likely to remove a proportion of the biotope and change the biological character of the biotope. Therefore, resistance is recorded as ‘Low’, resilience is recorded as ‘Medium’ and sensitivity is assessed as ‘Medium’.